The present invention relates to a coil apparatus for transmitting/receiving an electromagnetic wave, and a nuclear magnetic resonance imaging apparatus (hereafter referred to as “MRI apparatus”) using the coil apparatus.
The MRI apparatus irradiates a subject disposed in a uniform static magnetic field generated by a magnet, with an electromagnetic field, excites nuclear spins in the subject, then receives a nuclear magnetic resonance signal which is an electromagnetic wave generated by nuclear spins, conducts imaging of the subject. The irradiation with the electromagnetic wave and reception of the nuclear magnetic resonance signal are conducted by an RF coil which transmits or receives an electromagnetic wave having a radio frequency (RE). Transmission coils, reception coils or coils serving as both transmission coils and reception coils having various shapes suitable for the MRI apparatus have been developed.
In recent years, the MRI apparatuses have had higher magnetic fields. As its merit, sensitivity improvement, consequent shortening of imaging time, and improvement of the imaging resolution are anticipated. On the other hand, a higher magnetic field in the MRI apparatus causes new problems such as uneven current distribution in the RF coil, a loss increase caused by higher resistance in the RF coil, uneven excitation region caused by RF absorption in the human body, and lowering in coil productivity caused by a limit of the self-resonant frequency in the RF coil.
In order to solve the problem of the productivity lowering of the RF coil among the above-described problems, new schemes of the RF coil such as the microstrip line type and the multiple patch resonator type have been proposed (see, for example, US 2004/001239A1, and U.S. Pat. No. 5,557,247). They are attracting attention as substitute techniques for the conventional schemes such as the ring coil array and the bird cage type coil in the high magnetic field MRI apparatus.
The problem of the RF absorption and reflection in the human body can be solved by sweeping the gradient magnetic field strength when irradiating with the excited RF and irradiating with an optimized RF. In this method, however, there is a problem that the irradiation time becomes longer. On the other hand, a technique of shortening the irradiation time by using a large number of RF coils has been developed. This technique is called parallel RF transmission. The parallel RF transmission is a method obtained by applying an imaging time shortening technique (called parallel imaging) using reception coils having different sensitivity regions to RF transmission. The parallel RF transmission has been proposed as a countermeasure to the problem of the uneven excitation region caused by RF absorption in the human body.
For conducting the parallel RF transmission, a plurality of transmission RF channels having different sensitivity regions are needed. In general, a coil of microstrip line type can have a configuration of multi-channel transmission/multi-channel reception. Therefore, the microstrip line coil is suitable for the parallel RF transmission scheme. For conducting the multi-channel reception, however, a multi-channel receiver is needed. It requires some cost to prepare the multi-channel receiver. On the other hand, the RF coil of the multiple patch resonator type mentioned above is a coil corresponding to one-channel transmission/one-channel reception or two-channel transmission/two-channel reception called quadrature. The coil of this type has a comparatively simple configuration, and the coil of this type can be easily mounted on an MRI apparatus having only one channel of transmission and reception system. However, the coil of this type cannot cope with the multi-channel transmission and the multi-channel reception.
As an RF coil suitable for the high magnetic field MRI apparatus, therefore, a coil that makes it possible to choose and switch the suitable number of channels at the time of each of transmission and reception according to the apparatus configuration is desired. If a microstrip line type coil is used and as many transmission reception changeover switches, phase shifters and branching filters as the number of channels are used, it is possible in principle to form a configuration of multi-channel transmission/one-channel reception or a configuration of one-channel transmission/multi-channel reception. When the number of channels increases, however, there is a demerit that the circuit adjustment and the work of balancing the compounding conducted among channels become complicated.
A coil apparatus that makes the one-channel transmission/multi-channel reception possible is disclosed in Proceedings of International Society of Magnetic Resonance in Medicine, vol. 13, p. 954, (2005). In this coil apparatus, a multiple patch resonator type coil is used as a one-channel coil at the time of transmission and used as a multi-channel loop coil array at the time of reception. Even if this technique is used, however, adaptation to the multi-channel transmission/one-channel reception is difficult. Furthermore, since the reception coil scheme is a loop coil array, coupling between adjacent coils increases, resulting in a demerit that the mounting becomes difficult and the configuration becomes complicated. Furthermore, in this method, nothing but a multi-channel configuration of N/2 channels can be implemented, where N is the number of conductors in a multiple patch resonator type coil called rung.